Side impact- and turn-related crash rates are lowest at intersections where average lane widths are between 10 and 10.5 feet, according to a study presented at the Canadian Institute of Transportation’s annual meeting last month. This challenges the long-held, but often disputed, assumption that wider lanes are safer.

The study looked at vehicle-to-vehicle crashes at 70 signalized intersections in Toronto and 190 in Tokyo over periods of four to five years. Crash rates were highest where average lane widths at the approaches were narrower than 10 feet or wider than 10.5 feet. Intersection approaches with 10-foot lanes also carried the highest traffic volumes. Bicycle and pedestrian volumes generally increased as lanes became narrower. There was no significant difference in truck volumes.

Narrower lane widths (10 to 11 feet) are sanctioned in national policies outlined by AASHTO, particularly for urban areas, but the official standards in many states prohibit them. According to a 2010 study published in the ITE Journal, six states require a minimum of 12-foot lanes and another 24 states require 11-foot lanes.

The author of this most recent study notes that lane width guidelines, in particular, were established well before we had reliable crash and safety data. His work and other work cited in his paper show that the science behind many of those early assumptions is shaky. Fortunately, new research like this can help support a shift toward “substantive safety,” based on empirical evidence, rather than “nominal safety,” which assumes that design guidelines ensure safe outcomes—a topic that is covered in depth in ITE’s recent publication, Integration of Safety in the Project Development Process and Beyond: A Context Sensitive Approach.